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Elektor Electronics 11/98
A rock-solid RF signal with an accu-rately known frequency and level is amust for anyone seriously involved inrepairing radio receivers and othercommunications equipment like filtersand even antennas. In particular,
receiver RF input and IF (intermediatefrequency) sections can not be testedwith any degree of certainty if a trust-worthy RF signal generator is not tohand. Unfortunately, professional-grade RF signal generators (like themighty Hewlett Packard 8640B in ourdesign lab) cost an arm and a leg, evenin the surplus trade. None the less, youwill see at least one RF signal generator,home-made, thrown together fromother bits and pieces, or ex-MOD, inthe shack of the more advanced radioamateur, simply because this piece oftest gear is as indispensable as the plainold multimeter.
The stability of the RF signal gen-erator described in this article is suchthat it will meet the (moderate)demands of many amateurs. Offeringa frequency range of 0.5 through30 MHz and an output level down to–80 dBm, it is perfect for testing andaligning many receivers and their sub-circuits like RF/IF amplifiers, mixers
An RF signal genera-tor is used for repair-ing radio/TV circuits,
checking filters, align-ing receivers, and forcomparative sensitiv-
ity tests on all kinds ofreceivers, whether
home-made, restoredsurplus or off-the-
shelf. The generatordescribed here has an
output frequencyrange of 0.5 to
30 MHz, making itsuitable for many
applications.
10
Design by Guido Brunner
RF signal generator
RADIO, TELEVISION & VIDEO
Main specificationsà Frequency range: 0.5 MHz to 30 MHzà Output level: 0 dBm down to –79 dBm in 1-dB stepsà Max. output level: 0.63Vpp into 50 Ωà Output impedance: 50 Ωà AM inputà FM inputà LCD readoutà Microprocessor controlledà Optional serial interface
part 1: circuit descriptions
and demodulators.What requirements can be men-
tioned in relation to an RF signal gen-erator? The answer is very simpleindeed: you need to be sure of (1) thefrequency and (2) the level of the signalyou feed into the circuit (receiver)under test. If either of these is unreli-able, all testing and comparing ofreceiver specs becomes meaningless.In the present design, frequency sta-bility is assured by a PLL (phase-locked loop), while the output level isdetermined by a switched pi (pi) atten-uator, all under the control of a micro-processor.
B L O C K D I A G R A MBecause the actual circuit diagrams ofthe four modules that make up the sig-nal generator are a fairly complex lot
when presented together, it wasdecided to draw and dis-
cuss them as separateblocks. The
basic interac-tion ofthese blocksis illustratedin Figure 1.The blockd i a g r a mshows thatthe heart ofthe circuit isa PLL syn-thesizer mod-ule keeping aVCO (volt-
age-controlledoscillator) in check. The
VCO output signal is amplified andfed to the generator output as well tothe synthesizer input and the input ofthe attenuator. The PLL obtains digitalinformation on the target VCO fre-quency from a microprocessor module.The micro also takes care of the front-panel mounted user interface, whichconsists of 3 switches, a rotary encoderand an LCD (liquid-crystal display). Italso controls the amount of attenuationat the generator output, across a rangeof –1 dB through –79 dB. An optionalserial interface is available to enable theRF Signal Generator to be linked to aPC using an RS232 cable. Functionally,the instrument is completed by aninternal power supply.
P L L B O A R DThe circuit diagram of this first moduleto be discussed in detail is shown inFigure 2. It comprises three sub-cir-cuits: VCO, synthesizer and outputbuffer. The VCO and the synthesizertogether from the PLL.
VCO and buffersThe active element in the oscillator is adifferential amplifier built around tran-sistors T1, T2 and T3, whose gain
depends on the currentpassed by T3. The actualresonating element inthe oscillator is an L-Cparallel tuned circuitconnected to the inputof the difference amplifier. The LC net-work consists of inductors L1-L5 incombination with variable-capacitancediodes (varicaps) D9 and D10. Theother input of the oscillator isgrounded for RF by capacitor C10.Depending on the desired frequencyrange, one or more inductors areswitched into the oscillator. This isdone by pulling the non-commonedterminals to RF ground using +5Vcontrol voltages on PIN diodes D2, D4,D6 and D8. In the highest frequencyrange, all inductors are effectively con-nected in parallel. This is necessary tomake sure that the non-selected induc-tors and their parasitic capacitance cannot form a series tuned circuit thatwould prevent the oscillator fromoperating at the desired frequency. Allinductors are off-the-shelf miniaturechokes. The frequency range switchingtakes place at 1.024 MHz, 2.304 MHz,5.376 MHz and 13.056 MHz.
Capacitor C8 provides the neces-sary amount of positive feedback in theoscillator. An AF signal may be appliedto the emitter of T4 to effect amplitudemodulation (AM). Frequency modula-tion (FM) is also possible by superim-posing an AF signal onto the varicaptuning voltage. Although FM willcause the PLL to drop out of lock, theaverage frequency remains constantbecause the time constant of the con-
trol loop is not capableof tracking the ‘insta-bility’ caused by themodulation signal.
To make sure it isnot too heavily loaded,
the oscillator signal is first buffered by aFET (field-effect transistor), T4. Nextcomes the real amplifier, IC1, a typeNE592 which some of you may knowfrom baseband-video amplifiers insatellite-TV receivers. The amplifier isbiased at half the supply voltage byopamp IC3b, and its gain is defined byseries network R26-L8. Because of theinductor action, the gain decreases athigher frequencies. Because the VCOstrives to maintain a stable outputlevel, less gain on the NE592 automat-ically more gain in the differential oscil-lator. This purposely-created effect isessential for reliable starting of theoscillator at higher frequencies.
The NE592 being a differentialamplifier, it has two inputs, but alsooutputs. Both are used here. The signalat the first output (pin 7) is applied toemitter follower T5 which supplies theactual generator output signal at animpedance of 50 Ω (the standard in RFtest equipment). The other output sig-nal supplied by the NE592 is used todrive two sub-circuits. One branchgoes to the PLL chip via C23 and R33,the other is used to drive a voltage rec-tifier/doubler, D11-D12 which in turndrives amplitude-control opamp IC3a.The desired highest output amplitudemay be set using preset P1. The authorused a setting where 0 dBm (decibel-
11Elektor Electronics 11/98
Visit our Web site at http://ourworld.compuserve.com/homepages/elektor_uk
VCO
PLL
PSU
Interface
PC
30V
12V
5V
mains
keysF
Attenuator0 ... - 79 dB
LCD
980053 - 11
0.5 ... 30 MHz
Figure 1. Block dia-gram of the RF SignalGenerator. All intelli-gence is vested in amicrocontroller.
1
B2
R1
D1
1N4148 D2
BA243
C2
33n
L2
100µH
B3
R3
D3
1N4148 D4
BA243
C3
33n
L3
22µH
B4
R5
D5
1N4148 D6
BA243
C4
33n
L4
3µH9
B5
R7
D7
1N4148 D8
BA243
C5
33n
L5
0µH56
C8
330p
C9
330p
C13
100n
C10
33n
C15
27p
R13 R14 R15 R16 R19
R20 R21
R25
R24
R27
R29
R28
R31
R11
R17
R34
T1
BF494
T2
R41
T3
BF494
C40
100n
C12
10µ63V
C14
180p
T4
BF256B
C16
330p
L6
39µH
L7
39µH
R22
10k
R23
10k
C17
33n
L8
3µH3
R26
C214n7
R30
T5
2N5179
C22
33n
R32
47Ω
C11
68p
C19
100n
C20
47µ16V
C18
10µ16V
OUT
D10
BB130D9
R2
390Ω
R4
390Ω
R6
390Ω
R8
390Ω
2x
R10
330kR9
C6
2n2
FM
L1
330µH
C1
33n
R12
10k
C7
220n
AM
R18
100k
R37
10k
R36
10k
R35
R33
2
3
1IC3a 2k2
P1
C28
2µ216V
C26
2µ2 16V
C25
2n2
C24
330p
D12AA13
D11
AA113
6
5
7IC3b
C23
330p
R40
1kC
B
A
NE592
IC1
G2B G2A
G1B G1A
14
10
11
12
I2
I1
Q2
Q1
1
8
7
3
54
C30
2n2
C31
10n
C29
10n
C37
330n
C39
100nC36
1µ
C35
100nR39
18k
R38
R42
C34
100µ 10V
C38
10µ63V
LOCK
C33
40p
X1
4MHz
2x
A A
B B
C
D
E
F
G
H
I
J
J
K
(N14)
L
L
M
N
+30V
30V
+12V
12V
+5V
5VDLEN
SDA
SCL
U TUNING = 0 ... 30V
LOCK
PLL
8
4
IC3
IC3 = LM358P
12V U+
U+
5V
C32
47µ16V
A 11V9
B
C
D
E
F
10V2
0V
9V5
0V02
0V86
G
H
I
J
K
L
M
N3V5
4V2
0V35
5V9
5V1
K
9V
2V5
1V95980053 - 12
30V
C27
100n
SAA1057IC2
XTAL
TEST
AMIN
DLENFMIN
OUTSDA
SCL
DCSDCA
TCA TCB
IN
15
17
18
11
12
14
13
10
TR
16
CC
5
6
41 2 3
1
9
2 3
7
V
8
2
D1
1N4148
C1
100n
R3
6Ω81
R4
39Ω2
RE1
R1 R2 R5 R6
D2
1N4148
C2
100n
R9
368Ω
R10
12Ω1
RE2
R7 R8 R11 R12
D3
1N4148
C3
100n
R15
3k65
R16
24Ω3
RE3
R13 R14 R17 R18
D4
1N4148
C4
100n
R21
909Ω
R22
56Ω2
RE4
R19 R20 R23 R24
D5
1N4148
C5
100n
R27
3k92
R28
162Ω
RE5
R25 R26 R29 R30
D6
1N4148
C6
100n
R33
3k92
R34
162Ω
RE6
R31 R32 R35 R36
D7
1N4148
C7
100n
R39
3k92
R40
162Ω
RE7
R37 R38 R41 R42
D8
1N4148
C8
100n
R45
3k92
R46
162Ω
RE8
R43 R44 R47 R48
A1– 1dB
A2 A3 A4– 2dB – 4dB – 8dB
A5 A6 A7 A8– 16dB – 16dB – 16dB – 16dB
980053 - 13RE1 ... RE8 = V23042-A1001-B101
3
milliwatt) into 50 equals 0.63 Vp p a tthe generator output.
S y n t h e s i z e rThe circuit of the synthesizer largelyfollows the Application Note for theS AA1057 as supplied by Philips Semi-conductors. Some component values
in the control loop had to be modified alittle to optimise the behaviour of thePLL. The ‘LOCK’ output is only pro-vided for test purposes. The SAA 1 0 5 7receives its control information in I2Cf o rmat via its SDA, SCL and DLENinputs. These lines are connected to am i c r o c o n t r o l l e r. Basically, the SAA 1 0 5 7compares the frequency of the VCOwith that of a reference signal derivedfrom the ex t e rnal 4-MHz quartz cry s-tal. For this purpose the VCO signal isi n t e rnally divided by a factor deter-mined by the microprocessor. The fre-quency difference produces an err o rsignal which is converted into a corr e-
sponding varicap control voltage. Thiscontrol voltage is integrated by R40-C37 and has a range of 0-30 V. Remark-a b l y, the SAA1057 does not require anex t e rnal level converter for the varicapcontrol voltage — a special amplifier isincluded on the chip for this purpose,as well as a direct connection for +30 V( p i n7 ) .
Trimmer C33 allows the generatoroutput frequency to be calibratedagainst a frequency standard.
The VCO/PLL board requires threesupply voltages: +5 V for the synthe-s i z e r, +12 V for the VCO, and +30 Vfor the varicap voltage.
Figure 2. Circuit diagram ofthe VCO/PLL board. The heartof the PLL is an I2C-controlledsynthesizer chip typeSAA1057.
C1
1µ 16V
R1
K1P1
10k
R5 8x 10k 1
23456789
C5
100n
R2 R3 R4
R28
1k
PSEN
ALE
R29
3k3
T12
R26
1k
R27
3k3
T11
R24
1k
R25
3k3
T10
R22
1k
R23
3k3
T9
R12
1k
R13
3k3
T4
R10
1k
R11
3k3
T3
R8
1k
R9
3k3
T2
R6
1k
R7
3k3
T1
R20
1k
R21
3k3
T8
R18
1k
R19
3k3
T7
R16
1k
R17
3k3
T6
R14
1k
R15
3k3
T5
X1
11.059 MHz
C2
33p
C3
33p
C4
100n
S1S2S3
T0
MAX232
R1OUT
R2OUT
RS1OUT
RS2OUT
IC2
T1IN
T2IN
RS1IN
RS2IN
C1–
C1+
C2+
C2–
11
12
10
13
14
15
16V+
V-
7
8 9
3
1
4
5
2
6
C9
C10
C6
C7
C8
K2
1
2
3
4
5
6
7
8
9
S4
ENCODER
A1
A2
A3
A4
A5
A6
A7
A8
C
B
A
P2.4
EA/VP
ALE/P
RESET
89C51
P0.0
P0.1
P0.2
P0.3
P0.4
P0.5
P0.6
P0.7
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
P2.0
P2.1
P2.2
P2.3
P2.4
P2.5
P2.6
P2.7
PSEN
INT0
INT1
IC1
TXD
RXD
39
38
37
36
35
34
33
32
21
22
23
24
25
26
27
28
31
19
X1
18
X2
20
40
17 RD16 WR
29
30
11
1012
13
14T015T1
1
2
3
4
5
6
7
8
9
B5
B4
B3
B2
DGCLK
DGDIR
C12
100n
C11
220µ16V
5V
T1 ... T8 = BC557B
C6 ... C10 = 10µ / 63V
OUT
IN
RS232
RES
5V 5V 5V
LOCK
980053 - 14
+5V
RS
R/W
EN
DB4
DB5
DB6
DB7
5V
5V
5V
T9 ... T12 = BC557B
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
5V
TxD
RxD
SDA
SCL
DLEN
Figure 4. Circuit diagram ofthe controller board. An 89C51sits between a number ofinput and output devices.
4
Figure 3. Circuit diagram ofthe digitally controlled attenu-ator. Range is –1 dB to –79 dBin 1-dB steps.
A T T E N U A T O R B O A R DFigure 3 shows the circuit diagram of adigitally controlled 8-section pi RFattenuator with a range of –1 dB to–79 dB in 1-dB steps. The resistor com-binations we need to realize each ofthe 79 discrete attenuation levels areconnected into the circuit by means ofrelay contacts. The associated relays areactuated and de-actuated by micro-processor drive signals that form 8-bitcombinations at the control inputsmarked A1-A8.
The theoretical values of the resis-tors in the attenuator are realized bymeans of parallel combinations of 1%resistors from the E96 series.
Each relay coil is shunted by a back-emf suppressor diode and a decou-pling capacitor.
M I C R O C O N T R O L L E RB O A R DAll the intelligence we need to imple-ment a man/machine interface, i.e.,establish communication between theuser on the one hand, and the PLL andthe attenuator on the other, is packedin a microcontroller type 89C51. Thiscontroller executes a program written
by the author and burned into theinternal program memory by the Pub-lishers. The 89C51 is available ready-programmed from the Publishers orcertain kit suppliers advertising in thismagazine.
The 89C51 accepts information andsupplies information. Microcontrollerfreaks call this ‘I/O’ for input/output.Well, the input devices are a rotaryshaft encoder, S4, which is used for thefrequency setting, a small keyboard,S1-S2-S3, the SDA line of the I2C busand (optionally) the RxD line of theMAX232 serial interface. The outputdevices to control are the LCD con-nected to port P0, the attenuator onport P1), the VCO inductors on portline P2.0 through P2.3 and, of course,the synthesizer chip, by way of theDDA and SCL lines (P2.6 and P2.7).Actually, the I2C bus is modified into aso-called CBUS by the addition of P2.5(DLEN) and its pull-up resistor, R2.
The 89C51 is clocked at11.0592 MHz by an external quartzcrystal, X1. This frequency was chosenbecause it allows standard baud ratesto be used on the serial interface.
A classic power-on reset network,R1-C1, completes the microcontrollercircuit.
This board requires only +5 V to
operate, the MAX232 having on-chipstep-up converters for +10 V and–10 V.
P O W E R S U P P L Y B O A R DAs you can see from the circuit dia-gram in Figure 5, the power supply forthe RF signal generator is entirely con-ventional.
The 30-V varicap supply is based ona simple combination of a zener diodeand a series transistor. Current drainon the 30-V rail will be very small, soextensive regulation is not necessary.None the less, a fair number of decou-pling capacitors is used to keep the var-icap voltage as clean as possible. Afterall, all hum, noise etc. on this rail willcause frequency modulation on theoutput signal. The input voltage for the30-V regulator is supplied by a voltagedoubler, C10-D5-D5.
The 5-V and 12-V supplies are basedon two old faithfuls, the 7805 and theLM317 respectively. These ICs andtheir usual ‘satellite’ components havebeen used so many times in our pub-lished circuits that no further descrip-tion will be necessary.
A single mains transformer rated at15 V, 8VA, supplies all the necessaryalternating voltages. The mains voltageat the primary side is applied via adouble-pole switch and a fuse, bothbuilt into a Euro-style appliance socket.
N E X T M O N T HIn next month’s second and conclud-ing instalment we will be discussingthe construction of the instrument onfour printed circuit boards. The articlewill be concluded with notes on theoperation of the RF Signal Generator,miscellaneous matters and optionalextras.
(980053-1)
15Elektor Electronics 11/98
D3
D1
D2
D4
15V
TR1
8VA
C3
C4
C2
C1
7812
IC1
IC2
LM317T
C5
1000µ 35V
C7
2µ2 16V
C9
2µ216V
C13
10µ 63V
C12
1µ63V
C11
220µ63V
C10
470µ63V
K1K2
C6
220n
D5
C8
220n
R1
22
Ω
5W
R2
27
0Ω
R3
82
0Ω
D6
1N4001
R4
1k
D7
33V
400mW
R5
10
k
D8T1
BC141+30V
+12V
+5V
30V
12V
5V
2x
63mA T
980053 - 15
D1 ... D4 = 4x 1N4001C1 ... C4 = 4x 47n
Figure 5. Circuit dia-gram of the powersupply. Three voltagesfrom one transformer!
5
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Elektor Electronics 12/98
The RF signal generator is a quite com-plex instrument, and we should reallyadvise beginners not to attempt tobuild this project without the help orguidance of someone with consider-able experience in building RF andmicrocontroller circuits.
There are no fewer than four boardsto build up, and each of these boardscontains a fair number of components.Add to that the mounting of the fourboards in a case and the inter-board
Although the main subject of this month’ssecond and final instalment is ‘all matters
constructional’, there’s also information onadjusting the instrument and, of course, on
how to use it!
20
Design by G. Brunner
RF signal generatorpart 2 (final): construction,operation and adjustment
RADIO, TELEVISION & VIDEO
Figure 6. Copper tracklayout and componentoverlay of the powersupply board.
(C) ELEKTOR980053-4
C1
C2
C3
C4
C5
C6 C7
C8C9
C10 C11
C12
C13
D1
D2
D3
D4
D5
D6
D7
D8
IC1
IC2
K1
R1
R2
R3
R4
R5
T1
TR1
0 +30V +5V+12V
980053-4
~
~
(C) ELEKTOR980053-4
wiring, and you are looking at a projectwhich should take even advanced hob-byists several hours, winter evenings orrainy Sunday afternoons to complete.
The four boards are built up one byone in the order indicated by the textto follow. As usual, great care should betaken to fit each and every part in theright position on the board. The com-ponent overlays and associated partslist should guide you through theprocess of assembling the boards. Par-ticularly with the 1% resistors in theattenuator section, you should (1)ascertain the value and (2) look up theposition on the board, before (3) fittingany resistor.
P O W E R S U P P L Y B O A R DThis board is the simplest to build. Pop-
ulating it should be straightforward,using the relevant Components Listand the component overlay shown inFigure 6. Resistor R1 may run fairly hotand should not touch the circuit board.The LM317T voltage regulator may bemounted directly on to the heatsink —an insulating washer is not required.The ‘power on’ LED is not fitteddirectly on the board — instead, it isconnected up via a pair of thin wireswith an length of about 20 cm.
This board is simple to test by pro-visionally connecting it to the mainsand using a voltmeter to check theindicated output voltages: +5 V, +30 Vand +12V. The finished PSU board isshown in Figure 7. Check your workagainst this photograph!
C O N T R O L L E R B O A R DThe controller boardshown in Figure 8 is far
more densely populated than the PSUboard. Hence, great care and precisionis required when it comes to solderingthe parts in place.
Start with the two wire links on theboard — you’ll find them near presetP1. Next, fit the components, the bestorder is probably from low-profileparts (resistors, IC sockets) to uprightmounted parts (crystal, transistors,radial electrolytic capacitors).
The three push-buttons, S1, S2 andS3, are not mounted directly on to theboard. Their pins are inserted in socketstrips or stacked IC sockets so that theirheight can be adjusted a little. Alterna-tively, their pins are ‘lengthened’ usingpieces of stiff wire. This is necessary toenable the cap tops to protrude a littlethrough the front panel. The samemounting method is used for LCD. As
with the push-buttons,the height of the LCD
21Elektor Electronics 12/98
Visit our Web site at http://www.elektor-electronics.co.uk
980053-3(C) ELEKTOR
C1
C2
C3
C4C5
C6
C7
C8
C9
C10
C11
C12
H1
H3
H4
H5
H6
H7
H8
IC1 IC2
K1
P1
R1
R2
R3R4
R5
R6R7R8R9
R10R11
R12R13R14R15R16R17R18R19R20R21
R22
R23
R24
R25
R26
R27
R28
R29
S1 S2 S3
S4
T1
T2
T3
T4
T5
T6
T7
T8
T9 T10 T11 T12
X1
0
+5V
RES
C BA
T0
P2.4
ALE
PSenLock
T
out in
980053-3
A8A7A6A5A4A3A2A1
B2 B3 B4 B5
RS232
Figure 7. Finished PSUboard (prototype).
Figure 8. Controllerboard artwork.
7
COMPONENTS LIST
POWER SUPPLY BOARD
Resistors:R1 =22Ω 5WR2 =270ΩR3 =820ΩR4 =1kΩR5 =10kΩ
Capacitors:C1-C4 =47nFC5 =1000µF 35V radial
C6,C8 =220nF MKTC7,C9 =2µF2 16V radialC10 = 470µF 63V radialC11 = 220µF 63V radialC12 = 1µF 63V radialC13 = 10µF 63V radial
Semiconductors:D1-D6 = 1N4001D7 = 33V 400mW zener diodeD8 = LED, red, high efficiencyT1 = BC141IC1 = 7812
IC2 = LM317T
Miscellaneous:TR1 = mains transformer, 15V 8VA,
Monacor/Monarch type VTR8115K1 = PCB terminal block, 2-way, raster
7.5mmK2 = mains socket, integral switch and
fuseholder, with fuse 63mATHeatsink type SK59 37.5mm (Fischer,
Dau Components)PCB, order code 980053-4 (see Read-
ers Services page)
above the controller board may needto be adjusted later, so do not mount itsecurely as yet. The rotary switchencoder, S4, is mounted directly on tothe board, but its spindle is not yet cutoff. Later, rectangular clearances are cutin the front panel to allow the LCD tobe viewed, and the push-buttons to bepressed.
It is recommended to use socketsfor IC1 and IC2. All holes in the PCBwith a label printed near it (like A1, T0,Psen, Lock, etc.) are for inter-boardwires. Solder pins are not strictly nec-essary — direct wire connections to theboard are also fine. As with the PSUboard, check your work against ourfully working prototype. This time,refer to the photograph in Figure 9.The board is fitted vertically behind themetal front plate (which has to be pur-chased separately). It is held in positionby a pair of slots moulded on the bot-
tom plate of the case. Several slots areavailable, and the pair you actuallychoose to use should ensure that themetal frame around the face of theLCD is pressed firmly against theinside of the front panel. The threetype ‘D6’ push-buttons should thenprotrude a little from the front panel.
The holes marked ‘In’, ‘Out’ and‘ground’ to the right of preset P1 arefor an optional 3-wire RS232 link to aPC. If you do not require PC control,the MAX232 may be omitted. The prac-tical use of the RS232 interface will bereverted to further on.
V F O / P L L B O A R DAs you can see from the PCB artworkin Figure 10, this is the board with thehighest component density of all four.Care and precision are essential if youwant to avoid a tedious faultfindingsession. Identify and check each part
before fitting it, and double-check itsvalue and position using the Compo-nents List and the component overlay.
As usual, start with the wire links(there are three), so they are not for-gotten or overlooked. Then follow thelow-profile parts and, finally, the verti-cally mounted parts. IC sockets shouldnot be used for the NE592 and theSAA1057 on this board.
The value of the inductors is usuallyprinted on these parts in the form ofcolour bands (like resistors) or dots.
The PLL/VFO board is fitted in atinplate enclosure from Teko. After thesolder work, inspect the board, andcompare yours with our prototypeshown in Figure 11.
22 Elektor Electronics 12/98
COMPONENTS LIST
CONTROLLER BOARD
Resistors:R1 = 22kΩR2,R3,R4 = 4kΩ7R5 = 10kΩ 8-way SIL arrayR6,R8,R10,R12,R14,R16,R18,R20,R22,R
24,R26,R28 = 1kΩR7,R9,R11,R13,R15,R17,R19,R21,R23,R
25,R27,R29 = 3kΩ3P1 =10kΩ preset, H
Capacitors:C1 =1µF 16V radialC2,C3 =33pFC4,C5,C12 =100nF ceramicC6-C10 =10µF 63V radialC11 = 220µF 16V
Semiconductors:T1-T12 = BC557BIC1 =AT89C51-20PC or
SC87C51CCN40 (order code 986515-1)
IC2 = MAX232
Miscellaneous:X1 = 11.059MHz crystalS1,S2,S3 = pushbutton, 1 make con-
tact, ITT type D6-R-RD; cap type D6Q-RD-CAP (Eurodis)
K1 =14 way SIL pinheaderK2 =9-way sub-D socket (female)S4 = rotary encoder, Bourns type
ECW1J-B24-AC0024 (Eurodis)LCD, 2x16 characters, Sharp type LM
16A211 (Eurodis)PCB, order code 980053-3 (see Read-
ers Services page)
Figure 9. Finished con-troller board (proto-type).
980053-3(C) ELEKTOR
9
and the trimmer to the centre of theirtravel. It is assumed that the powersupply board has been tested already(with good results, of course).
After applying power, the first thingto do is set the LCD contrast with pre-set P1. Next, use an oscilloscope tocheck that the VFO/PLL board sup-plies an RF signal to the attenuatorboard.
The output frequency supplied bythe generator may be checked with acalibrated frequency meter, a fre-quency standard (off-air Rugby MSF orsimilar) or a calibrated SW receiver(zero-beat). The relevant adjustment istrimmer capacitor C33.
23Elektor Electronics 12/98
Visit our Web site at http://www.elektor-electronics.co.uk
980053-1
(C) ELEKTOR
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10C11
C12
C13
C14
C15 C16
C17
C18
C19
C20
C21 C22
C23C24
C25
C26
C27
C28
C29
C30C31
C32
C33
C34
C35
C36
C37
C38
C39
C40
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11 D
12
H1 H2
H3H4
IC1 IC2
IC3
L1
L2
L3
L4
L5
L6L7
L8
P1
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10R11
R12
R13
R14
R15
R16
R17
R18
R19R20
R21
R22
R23
R24R25
R26
R27 R
28R
29R
30
R31
R32
R33
R34
R35
R36R37
R38
R39
R40
R41
R42T1T2
T3
T4T5
X1
0
0
T T T
OUTAMFM
+12V
+5V
B5
B4
B3
B2
980053-1
LOCK
AB C
+30V
980053-1
(C) ELEKTOR
10
Figure 10. VFO/PLLPCB design.
A T T E N U A T O R B O A R DThe main point to mind about assem-bling the attenuator board (Figure 12)is that each close-tolerance (1%) resis-tor goes to the right position on theboard. One error in this respect maycause wrong attenuation levels later,with possibly difficult to explain behav-iour of some of the radio equipmentyou may be aligning! Our advice is,therefore: read the Components Listcarefully, check the colour code, use aDMM to measure the value of each
resistor, and then check its position onthe board.
The attenuator board has relativelylarge copper areas to assist in screeningand preventing unwanted signals frombeing generated and picked up by thecircuit. The attenuator board is shownin Figure 11, together with theVFO/PLL board. For RF screening pur-poses, both boards are fitted in Tekotinplate cases.
A D J U S T M E N TThe boards may be wired up experi-mentally for an initial test and a fewadjustments.
To begin with, set the two presets
COMPONENTS LIST
VFO/PLL BOARD
Resistors:R1,R3,R5,R7,R12,R22,R23,R31,R34,R36
,R37 = 10kΩR2,R4,R6,R8 = 390ΩR9,R14,R15,R21,R27,R33,R40 = 1kΩR10,R41 = 330kΩR11,R13,R16,R18 = 100kΩR17,R26 = 100ΩR19 = 2MΩ2R20 = 1MΩR24,R25,R35 = 22kΩR28,R29 = 3kΩ3R30 = 560ΩR32 = 47ΩR38 = 180ΩR39 = 18kΩR42 = 10ΩP1 = 2kΩ multiturn preset, H
Capacitors:C1-C5,C10,C22 = 33nF ceramic
C6,C25,C30 = 2nF2 ceramicC7 = 220nF MKTC8,C9,C16,C23,C24 = 330pF ceramicC11 = 68pF ceramicC12,C18,C38 = 10µF 63V radialC13 =100nF ceramic 5mmC19,C27,C35,C39,C40 =100nF ceramicC14 = 180 p ceramic C15 = 27 p ceramic C17 = 33n ceramic 5mmC20,C32 = 47µF 16V radialC21 = 4n7 ceramic C26,C28 = 2µF2 16V radialC29,C31 = 10nF ceramic C33 = 40pF trimmerC34 = 100µF 10V radialC36 = 1µF MKTC37 = 330nF MKT
Inductors:L1 = 330µHL2 = 100µHL3 = 22µHL4 = 3µH9L5 = 0µH56
L6,L7 = 39µHL8 = 3µH3
Semiconductors:D1,D3,D5,D7 = 1N4148D2,D4,D6,D8 = BA243D9,D10 = BB130D11,D12 = AA113T1,T2,T3 = BF494T4 = BF256BT5 = 2N5179IC1 =NE592N (N14)IC2 = SAA1057 (Philips)IC3 = LM358P
Miscellaneous:X1 = 4MHzTinplate case, Teko, size 160x25x49mmCase, Bopla type Ultramas UM52011
(size 224x72x199mm)Front panel type FP50011 or FPK50011PCB, order code 980053-1 (see Read-
ers Services page)
980053-2
(C) ELEKTOR
D1 D2 D3 D4 D5 D6 D7 D8
H1
H2
H3
H4
OU
T
R1
R2
R3R4
R5
R6
R7
R8
R9R10
R11
R12
R13
R14
R15R16
R17
R18
R19
R20
R21R22
R23
R24
R25
R26
R27R28
R29
R30
R31
R32
R33R34
R35
R36
R37
R38
R39R40
R41
R42
R43
R44
R45R46
R47
R48
RE1 RE2 RE3 RE4 RE5 RE6 RE7 RE8
A8A7A6A5A4A3A2
A1
980053-2
0
T T
C1
C2 C3 C4 C5 C6 C7 C8
980053-2
(C) E
LEK
TOR
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Figure 12. AttenuatorPCB artwork.
Adjustment of the RF signal level isonly possible if you have an accurateand calibrated RF voltmeter. With theattenuation set to 0 dB, preset P1 maybe adjusted for an output level of630 mVpp into 50 Ω at the generatoroutput. Failing the necessary testequipment, you may leave the multi-turn preset at mid-travel.
W I R I N G A N DM E C H A N I C A L W O R KAlthough there are quite a few wireconnections between the boards, thereare no special precautions in thisrespect. The RF signal connectionbetween the PLL/VFO board and theattenuator board must, of course, bemade in coax cable. The same goes forthe connections between the AM and
FM inputs on the PLL/VFO board andthe associated BNC sockets on thefront panel. If you can get hold of it,use the 3-mm dia. type RG174/U, else,the much thicker RG50/U or /CU is agood alternative.
All other inter-board connectionsare made in light-duty flexible wire orflatcable, although slightly thicker wireshould be used for the 0-V, 5-V and 12-
24 Elektor Electronics 12/98
11
Figure 11. Finished PLL/VFO board (below) and attenuatorboard (above), both fitted in ‘Teko’ ready-made tinplate cases.
V supply wiring. Do not make any ofthe wires longer than necessary to pre-vent digital noise being picked up fromthe controller board.
The wires and the coax cables toand from the PLL/VFO board and theattenuator board should pass throughholes drilled in the short side panels ofthe Teko tinplate cases. Once theseboards are fully operational, the topcovers are fitted for optimum RFscreening.
Guidance for mounting the fourboards into the Bopla enclosure maybe obtained from the photographs inthis article, and in particular, Figure 13.Note that the solder side of the powersupply board is protected by a perspexcover plate cut to roughly the samesize as the board. The VFO/PLL andattenuator boards are screened by tin-plate boxes, and mounted horizontally
on to the bottom plate of the enclosure.As already mentioned, the PSU boardis fitted vertically, using a pair of themoulded PCB slots towards the backpanel. The three holes at the ‘empty’right-hand side of the controller boardare drilled to a diameter of about 8 mmto allow the coax cables to the threefront-panel mounted BNC sockets topass.
The mains voltage is switched onand off by a double-pole switch inte-grated into a mains socket fitted ontothe plastic rear panel of the enclosure.The wires between the mainssocket / swi tchcombination andthe PCB terminalblock on the PSUboard should bemains-rated andproperly iso-
lated. At the PCB side in particular, the‘live’ and ‘neutral’ wires should not bestripped longer than strictly necessary,and they should be inserted into theclamps right up to the insulation.Finally, once the wires are connected,the terminals on the mainssocket/switch combination must beinsulated using heat-shrink sleeving.
The metal front panel is cut, drilledand lettered using the template shownin Figure 13. This front panel foil is notavailable ready-made.
In the (ABS plastic) back panel, youhave to cut rectangular clearances for
the mainssocket /switchc o m b i n a t i o nand, optionally,for the RS232connector (a 9-pin sub-D type).
25Elektor Electronics 12/98
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13
Figure 13. A look inside our pro-totype of the RF Signal Generator.The covers of the tinplate casesof the VFO/PLL board and theattenuator board were removedfor this photograph.
COMPONENTS LIST
ATTENUATOR BOARD
Resistors (all 1%):R1,R5,R21 = 909ΩR2,R6 = 20kΩR3 = 6Ω81R4 =39Ω2R7,R11 = 475ΩR8,R12 = 6kΩ19R9 = 368ΩR10 = 12Ω1
R13,R17 = 243ΩR14,R18 = 2kΩ74R15,R20,R24 = 3kΩ65R16 = 24Ω3R19,R23 = 121ΩR22 = 56Ω2R25,R29,R31,R35,R37,R41,R43,R47 =
75ΩR26,R30,R32,R36,R38,R42,R44,R48 =
825ΩR27,R33,R39,R45 = 3kΩ92R28,R34,R40,R46 = 162Ω
Capacitors:C1-C8 = 100nF SMD
Semiconductors:D1-D8 = 1N4148
Miscellaneous:RE1-RE8 = relay, 2 x change-over, type
V23042-A1001-B101 or V23042-A2001-B101 Siemens (Eurodis, Elec-troValue)
PCB, order code 980053-2 (see Read-
O P E R A T I O N
The instrument is controlled by meansof three pushbuttons and a rotarye n c o d e r, all accessible on the frontpanel. The instrument communicates
with you via an LCDwith two lines of 16c h a r a c t e r s .
The functions ofthe ‘left’ and ‘r i g h t ’pushbuttons are self-evident, we recko n ,because they movethe cursor on the LCdisplay in the direc-tion indicated by thea rrows on the frontp a n e l
From the startingposition (cursor on‘MHz’), the cursormay be moved to theleft on to any of thepost-decimal posi-tions of the fre-q u e n c y. The numberat which the cursora rrives may then bechanged by turn i n gthe rotary encoder.The frequency set inthis way is howevernot actually gener-ated until you pressthe ‘Enter’ pushbut-ton (asynchronousoperation, this isindicated in theupper right-handc o rner of the dis-play). After any fre-quency change, thePLL status is indi-cated by ‘lock’ in theleft-hand bottomc o rner of the read-o u t .From the initial
position to the right,the cursor jumps to‘M0’ (memory 0 ) .This indicates twomemories, M0 andM1, in which fre-quency and attenua-tor settings may bestored. You press theEnter key to changebetween these mem-ories. In this way,you can quicklychange between twopreviously storedsettings, which maybe useful, for ex a m-
ple, for aligning a filter. Altern a t i v e l y,you may use the same frequencytwice, but with two different attenua-tor settings. This facility is useful foradjusting, say, a receiver AGC (auto-matic gain control).
Moving further to the right, the cur-sor jumps on ‘asy’. Here you canswitch to asynchronous operation bypressing ‘Enter’. In synchronousmode, any frequency changerequested by way of the rotary
encoder is immediately passed on tothe VFO/PLL unit. In this mode, the RFoutput frequency is continuouslyadjustable, but only within the selectedrange (one of five). If you turn theencoder to a frequency outside a cer-tain range, the PLL will drop out oflock, and the ‘lock’ indication will dis-appear from the LCD. By pressing anyke y, the PLL is returned to asynchro-nous mode, and the last selected fre-quency is automatically restored. If youthen move the cursor to a decimal digitof the frequency readout, and press theEnter ke y, the generator changes to therelevant frequency range, allowing youto change to synchronous mode againand continue ‘tuning’ again using con-tinuous frequency variation.
One more position to the right, thecursor reaches the ‘dB’ position, indi-cating the currently valid attenuation.The desired attenuation may be setwith the aid of the rotary encoder. Aswith the frequency setting, thedesired attenuation becomes effectiveonly when you press the Enter ke y.This is done to reduce wear and tearon the relays.
O P T I O N A LR S 2 3 2 I N T E R FA C EThe RS232 interface on the controllerboard is an optional extension whosefunction has not been fully developedout by the author/designer. Basically, itwas designed into the circuit to enablethe generator frequency and outputsignal attenuation to be controlled by aP C .
The communication parameters areas follows: 9600 bits/s, 8 data bits, 1stop bit. The communication workswith character strings, and is easilytested with the aid of a terminal pro-gram. To set the frequency you have tosend an ‘F’ (for ‘f r e q u e n c y’), then fivenumbers for the frequency in kilo-hertz, and, finally, a carr i a g e - r e t u rn(CHR$(13)). An additional Line-Fe e d(CHR$(10)) will be ignored. If every-thing is correct (first character is ‘F’, atotal of 6 characters and the frequencyin the right range), the controllerr e t u rns a ‘D’ (for ‘done’), followed bya CR-LF sequence, otherwise, an ‘E’(for ‘err o r’) and a CR- L F.
The attenuation is set by sending an‘A’, two numbers and CR. Again thecontroller answers as described. Themain purpose of the serial interf a c ewas to create a basis for using the gen-erator in an environment like Lab-Vi e wT M.
( 9 8 0 0 5 3 - 2 )
1 4
Figure 14. Suggested front-panel layout. Use it as a tem-plate to drill the metal frontpanel of the instrument, andapply the lettering/symbols.
50Hz
No. 980053
240V
F = 63mA T
ELEKTOR
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